The invention relates to process control and, more particularly, to improved methods and apparatus for determining optimum gain settings for process control equipment.
"Process control" refers to the control of the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of a process do not vary substantially during a single run or over the course of several runs. Process control has application in both the manufacturing and service sectors.
A process control unit, or "controller," typically operates by monitoring and comparing a process characteristic, the controlled variable, with a desired "setpoint" level to determine whether the process is operating within acceptable bounds. As the controlled variable begins to deviate from the setpoint, the controller manipulates one of the process inputs, the manipulated variable, to bring the process back to the desired level of activity.
For example, as shown in FIG. 1, a process controller can oversee a process in which fluid flows at a constant rate from a continuously refilled tank. The controller monitors the liquid level in the tank and, when necessary to prevent the tank from running dry or overflowing, adjusts an inlet valve to increase or restrict the inflow.
Among the controllers developed by the art is the so-called proportional-integral-derivative (PID) controller shown in FIG. 2a. This generates the manipulated variable signal, m, as a predetermined mathematical function of the controlled variable signal, c. The output of such a controller can be expressed by the mathematical relation: ##EQU1## where, m is the manipulated variable;
e is the error (the difference between the controlled variable and the setpoint); PA1 P, I and D are, respectively, the proportional band, integral time constant and derivative time constant of the controller. PA1 K is the current gain of the automatic control subsection, PA1 OVS.sub.o is the observed overshoot, and PA1 OVS.sub..tau. is the predetermined overshoot. PA1 .tau..sub.1 is a primary time constant of that process; and PA1 .tau..sub.d is a dead time of that process. PA1 .tau..sub.2 is a secondary time constant of the process and PA1 .tau..sub.d is a dead time of the process. PA1 .tau..sub.1 is a primary time constant of the process PA1 .tau..sub.2 is a secondary time constant of the process, and PA1 .tau..sub.d is a dead time of the process.
The art has also developed controllers which operate by modeling the specific processes they control. This is in contrast to the PID controller, which generates the manipulated variable signal as a generalized function of the controlled variable signal. One such model-based controller is the dead time controller, which uses the process dead time and lag (as well as the controlled variable) to generate the manipulated variable.
Dead time is the time it takes a change in the manipulated variable applied to a process to be reflected by a change in the controlled variable generated by that process. Lag is the time, after the dead time period, that it takes the controlled variable to move approximately 63% of its final value, following a step change in the manipulated variable.
A dead time controller of the type referred to above is shown in FIG. 2b. It is constructed by adding a "dead time" element, i.e., a time delay, into the integral feedback loop of a PID controller. This controller is referred to by the mnemonic "PID.tau..sub.d."
To function properly, a process controller must be tuned to the process it controls. According to a prior art open-loop tuning method, controller settings are made in accord with values of parameters such as the process gain, K.sub.p, which reflects the magnitude by which the process responds to a step change in the manipulated variable. In accord with a conventional open-loop method, the operator applies a single step to the process, monitor its response and, from that, makes calculations necessary to determine the process parameters.
According to the prior art closed-loop tuning method, the controller is used to drive the process/controller loop into oscillation, and from that, process parameters are inferred. Under this method, the controller is typically placed in proportional mode and the band setting, P, is decreased until the loop begins to cycle. In that state, the process parameters are determined, for example, from the natural period of oscillation and the controller proportional band setting.
While the aforementioned methods are acceptable for tuning conventional PID controllers, they have not proven successful for model-based controllers and, particularly, controllers with dead time, such as PID.tau..sub.d controllers.
It is, accordingly, an object of this invention to provide improved systems for process control. More particularly, an object is to provide improved methods and apparatus for tuning process control equipment.
A further object is to provide improved methods and apparatus for tuning model-based controllers and, more particularly, dead-time controllers.